Solid-state optical wavelength switches

ABSTRACT

The present invention provides improved optical wavelength switches in which no mechanical movement is required to direct optical pathways between plural fiber ports. The inventive three fiber ports devices divide the incoming optical signals into two subsets of spectrums and selectively direct them into two output ports in response to an electrical control signal. In the inventive switch, an optical signal is spatially split into two polarized beams by a birefringent element, which pass through a series polarization rotation elements and recombine into output fibers, achieving polarization independent operation. Advantageously, the inventive switches incorporate two-stage polarization rotations to improve isolation depth, as well as temperature and wavelength independence. The inventive switches also incorporate light bending devices to allow two fibers to be coupled to the light beams using a single lens achieving small beam separation for compactness. The switches of the present invention rely on electro-magnetically or electro-optically switching the beam polarizations from one state to another to rapidly direct the light path.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is related to optical devices; moreparticularly, the invention relates to non-mechanical optical wavelengthselective switches.

[0003] 2. Description of Related Art

[0004] Fiberoptic wavelength division multiplexing (WDM) has emerged asthe dominant platform for telecommunications, providing a major leap incapacity by enabling a single fiber optic cable to transmit multiplewaves of light at once thereby multiply increasing communicationbandwidth. WDM systems transmit information by employing optical signalsof a number of different wavelengths, known as carrier signals orchannels. Each carrier signal is modulated by one or more informationsignals. For further bandwidth expansion, intelligent optical networksin which optical channels can be dynamically routed/switched in theoptical layer become critical. Therefore, wavelength selective opticalrouters/switches are a key component in the next-generation opticalnetworks, analogous to the electrical switches in electrical networks.Optical wavelength selective switches can be used to perform basic WDMfunctionalities, such as optical signal routing, channel add/drop, anddynamic multiplexing/demultiplexing. However, optical wavelengthselective switching has not been widely adopted because the lack ofcommercially available components of needed reliability.

[0005] In an optical switch, light signal must be accurately enteredinto an optical fiber, or much of the signal strength will be lost. Thealignment requirements of micro-optic devices are particularlystringent, as fiber core diameters are typically as small as 2 to 10micrometers and their acceptance angle is fairly narrow. Additionalinsertion losses reduce the amplitude of the optical signal. Therefore,optical switches which accept light from an input optical fiber, andwhich selectively couple that light to any of a plurality of outputoptical fibers must transfer that light with precise alignment andwithin the small acceptance angle for light to efficiently enter thefiber. Current optical wavelength selective switching are achieved bycoupling optical filters with mechanical optical switches, consequentlyhave drawbacks of slow, less reliable, and bulky. One such mechanicalwavelength selective switch is described in Lee U.S. Pat. No. 6,192,174.It is therefore greatly desirable to have integrated optical wavelengthselective switches that direct light beams according to their wavelengthwithout moving parts, a feature generally associated with highreliability and high speed.

[0006] A none-mechanical optical wavelength selective switch has beenproposed by Wu et al. FIG. 1 depicts a typical optical wavelength switch999 of the prior art as described in U.S. Pat. No. 5,694,233, issued toWu et al. on Dec. 2, 1997, which is incorporated herein by reference. AWDM signal 500 containing two different channels 501, 502 entersinterleaver 999 at an input port 11. A first birefringent element 30spatially separates WDM signal 500 into horizontal and verticallypolarized components 101 and 102 by a horizontal walk-off. Components101 and 102 are coupled to a two-aperture polarization rotator 40accordingly. The rotator 40 selectively rotates the polarization stateof either signal 101 or 102 by a predefined arnount to render theirpolarization parallel. The polarization rotator 40 consists of twosub-element rotators that form a complementary state, i.e. when oneaperture turns on the other turns off. By way of example, in FIG. 1signal 102 is rotated by 90° so that signals 103, 104 exiting rotator 40are both horizontally polarized when they enter a wavelength filter 61.

[0007] Waveplate-based wavelength filter 61 selectively rotates thepolarization of wavelengths in either the first or second channel toproduce filtered signals 105 and 106. For example wavelength filter 61rotates wavelengths in the first channel 501 by 90° but does not rotatewavelengths in the second channel 502 at all. The filtered signals 105and 106 then enter a second birefringent element 50 that verticallywalks off the first channel into beams 107, 108. The second channelforms beams 109, 110. A second wavelength filter 62 then selectivelyrotates the polarizations of signals 107, 108 but not signals 109, 110thereby producing signals 111, 112, 113, 114, having polarizations thatare parallel each other. A second polarization rotator 41 then rotatesthe polarizations of signals 111 and 113, but not 112 and 114. Theresulting signals 115, 116, 117, and 118 then enter a third birefringentelement 70. Third birefiingent element 70 combines signals 115 and 116,into the first channel, which is coupled to output port 14. Birefringentelement 70 also combines signals 117 and 118 into the second channelwhich is coupled into output port 13.

[0008] As described above, by suitably controlling the polarizationrotation induced by rotators 40 and 41, device 999 operates as awavelength selective switch. Furthermore, the wavelength selectiveswitch 999 can also operates as a passive interleaver multiplexer orde-multiplexer using a fixed sets of polarization rotators in 40 and 41.

[0009] Wavelength selective 999 has major drawbacks. First, Wu's switchis disadvantageously based on a large spatial separation between twofibers located on the same side. The configuration requires individualimaging lens for each fiber port and consequently needs large and longcrystals to deflect the beams. The use of three separated coflimators tocouple the signals into and out of optical fibers adds size, complexity,and cost. Moreover, the long coupling distance increases loss. The bulkysize also leads to in-stability, since the greater the mass ofbirefringent materials, the more unstable its operation. As a result,the optical wavelength switch 999 typically has large loss, excessivelylarge size, and is expensive to produce and less stable in operation.Second, the electrically controllable polarization rotators 40 and 41are based on a two-part-aperture design that rotates the optical beamsseparately in a complementary manner, i.e. when one turns on the otherturns off. Such design is primarily for incorporation of organic liquidcrystal device (LCD) based polarization rotators. Since LCD usuallyemploys surface electrodes in the light path to apply electrical field,consequently two individually controllable rotators can be easilyfabricated on a same element via electrode patterns. However, the use ofliquid crystal materials leads to undesirable properties of slow speedand large temperature dependence, which are obstacles for opticalnetwork applications. Recent progresses in inorganic magneto-optic andelectro-optic materials have opened new opportunities to producesolid-state optical switches of faster speed and high stability.However, the two-part separately controlled polarization rotator 40 and41 design in 999 is unsuitable for incorporating inorganic crystals.This is because it is very difficult and unpractical to apply twoopposite fields with reasonable uniformity to two adjacent Faradaycrystals or electro-optic crystals, due to the strong field interferenceacross the small spatial separation.

[0010] Recent version of optical interleaver as described by Li, U.S.Pat. No. 6,212,313 represents some improvement by using dual fibersharing a single imaging lens to reduce the optical device size.However, Li's wavelength selective devices are primarily designed forpassive interleaver applications. It thus has a disadvantage to bereconfigured as a active wavelength selective switch, since that it isbased on the same two-part-aperture polarization rotator design asdescribed in Wu's design. For reasons described above, Li's designs areunsuitable for wavelength switching/routing applications usingsolid-state materials of magneto-optic garnet or electro-optic crystalsas the controllable polarization rotators. Moreover, Li's reflectiontype optical configurations are based on use of either three separatedcollimators or a triple collimator on one side to couple the signalsinto and out of optical fibers. Using three individual collimatorssignificantly adds size and cost. Using a triple collimator substantialincrease complexity, resulting in increased interdependency among thealignments of each optical path. Therefore, manufacture of Li's devicesis difficult and the production cost is high.

[0011] Due to the above difficulties, solid-state wavelength switch hasnot yet been commercially available. There is a need, therefore, for animproved optical wavelength switch that overcomes the above drawbacks.It would be particularly desirable to provide optical wavelengthselective switches having low optical insertion loss and high speedswitching speed that is also reliable. Its is also important that theseswitches use components of small size and require reduced alignmentsteps with large assembly tolerance to facilitate low cost manufacture.The inventive optical devices described here provide these criticalattributes.

SUMMARY OF THE INVENTION

[0012] The present invention provides a compact and economicalnon-mechanical optical wavelength selective switch that can beefficiently coupled to optical fibers using fewer parts and having largeassembly tolerance. The inventive three fiber ports devices divide theincoming WDM optical signals into two subsets of channels and switchablydirect them into two output ports in response to an electrical controlsignal. The invention allows the use of inorganic crystals to achievefast and stable wavelength switching and filtering functions. Theinventive wavelength selective switches use at least one single lens tocoupling two fibers achieving small beam separation thus small size andlow material cost. The invention further consists of a light-bendingdevice, situated to compensate for the angle between the two light beamsthat share the same lens, advantageously increasing alignment tolerance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 depicts an isometric view of an optical wavelength switchaccording to the prior art;

[0014]FIG. 2 is an isometric view of a three-ports two-stage opticalwavelength selective switch according to a first embodiment of thepresent invention.

[0015]FIG. 3 is a plan view of a nonreciprocal optical wavelength switchof FIG. 2, and illustrates the arrangement of each element within theswitch body for this embodiment. FIGS. 3A and 3B are top view and sideview of the inventive switch, respectively.

[0016]FIG. 4 depicts cross section schematic views of polarization ofFIG. 3 after each component as the optical signal travels along theoptical paths, in accordance with the invention.

[0017]FIG. 5 depicts an isometric view of a reflection modenonreciprocal two-stage optical wavelength switch according to a secondembodiment of the present invention.

[0018]FIG. 6 depicts an isometric view of a reflection modebidirectional two-stage optical wavelength switch according to a thirdembodiment of the present invention.

[0019]FIG. 7 depicts an isometric view of a reflection mode passiveoptical wavelength interleaver according to a fourth embodiment of thepresent invention.

[0020]FIG. 8 depicts an isometric view of a reflection modebi-directional two stage optical light path switch according to a fifthembodiment of the present invention.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0021] The solid-state optical wavelength selective switch of thisinvention has several advantages over prior designs. First, theinventive configuration places two fiber ports on the same side to beclosely next to each other and to share the same inaging element,leading to fewer optical elements. The closely spaced beam propagationarrangement reduces the size requirement for each birefringent beamdeflection element, consequently lowering material cost. The design alsoresults in a smaller footprint of the devices. Prior non-mechanicaloptical wavelength switches have an arrangement wherein each opticalport has its own individual imaging element, disadvantageously requiringlarger and longer size of each component that comprises the device.Second, the design incorporates a beam angle correction system, allowingto adjust position and angular substantially independent, reducingposition sensitivity and achieving maximum light coupling. Thisinventive configuration greatly reduces the packaging difficulty,therefore, is particularly desirable for volume production. Third, theinventive switches are based on electrically controllable polarizationrotators of single aperture. This simple configuration is better suitedfor using magneto-optic Faraday crystals or inorganic electro-opticmaterials as the controllable polarization rotator. Prior non-mechanicaloptical wavelength switches have disadvantageous configurations whereinthe controllable polarization rotators comprise two-parts aperture ofdifferent rotations that is not amiable for using inorganic materials.

[0022] In one aspect of the invention, an optical signal of differentchannels may be rapidly and reliably switched between two optical paths,according to electrical control signals. The inventive opticalwavelength switch may be used in telecommunications systems/sub-systems,such as in WDM add/drop, multiplexers/demultiplexers, dynamicreconfiguration, signal routing. The inventive optical wavelengthselective switches are particularly suited for WDM optical networkapplications, where high speed and reliable switching is required. Theseand other advantages of the inventive optical switches are elaborated inthe specific embodiments now described.

[0023] The wavelength switch described here is a polarization-rotationbased device in which a randomly polarized input light beam is splitinto a pair of beams of two polarizations; its optical wavelength isfurther split into two sets of complementary spectrums of differentpolarizations by passing through waveplate-based filters; the lightbeams with on spectrum goes to one fiber but that with another spectrumgoes into another fiber. The electrically controlled polarizationrotators switch the state of polarization of the light beams from one toanother, consequently switch the two sets of wavelength from one port toanother port. The inventive device advantageously achieves routing whileconserving all optical energy regardless of the polarization of signals.

[0024] The inventive devices achieve wavelength selection by passinglight through at least one birefringent crystal filter. The principle ofits wavelength filtering function can be described as the following. Fora uniaxial crystal cut parallel to the optic axis, it introduces arelative phase difference Δδ between the two polarization components ofthe incident light wave. This phase shift can be expressed as:

Δδ(λ)=2π|n _(o)(λ)−n _(e)(λ)|L/λ  (1)

[0025] Where L is the crystal length, and n_(o) (λ) and n_(e) (λ) areits ordinary and extraordinary refractive indices, respectively.

[0026] When Δδ equals to 2kπ (k=0,1,2, . . . ), the relative retardationis one wavelength, the two polarization components are back in-phase,and there is no observable effect on the polarization of the incidentmonochromatic beam. However, when Δδ is equal to (2k+1)π (k=0,1,2, . . .), the effect of the crystal in the light path is to rotate thepolarized plane of the incident light by an angle between the incidentvibrations and the principle section. When the crystal's principle axisis oriented at an angle of 45° with the incident polarization plane, thevibration of the emerging light will rotate 90° with its originaldirection.

[0027] Since the phase shift is also a function of wavelength, with aparticular crystal length L, the birefringent crystal can introduce a2kπ (k=0,1,2, . . . ) phase difference to λ₁ as well as a (2k+1)π(k=0,1,2,) phase difference to λ₂ simultaneously. These L values can bedetermined by following equations: $\begin{matrix}\left\{ \begin{matrix}{\quad {{{\Delta \quad {\delta \left( \lambda_{1} \right)}} = {{2\pi {{{n_{o}\left( \lambda_{1} \right)} - {n_{e}\left( \lambda_{1} \right)}}}{L/\lambda_{1}}} = {{2\quad k\quad \pi \quad k} = 0}}},1,2,\quad \ldots}} \\{\quad {{{\Delta \quad {\delta \left( \lambda_{2} \right)}} = {{2\pi {{{n_{o}\left( \lambda_{2} \right)} - {n_{e}\left( \lambda_{2} \right)}}}{L/\lambda_{2}}} = {{\left( {{2\quad k} + 1} \right)\pi \quad k} = 0}}},1,2,\quad \ldots}}\end{matrix} \right. & (2)\end{matrix}$

[0028] Therefore, with a proper thickness and optic axis orientation, abirefringent crystal can selectively rotate the polarization of λ₂ by90° and at the same time maintains the polarization of λ₁, as a lightbeam containing λ₁ and λ₂ transmits through the birefringent crystalfilter. The effect of the birefringent waveplate filter for the incidentlight's entire wavelength spectrum is generating two eigen states. Thefirst eigen state carries a first sub-spectrum with the samepolarization as the input, and the second eigen state carries acomplementary sub-spectrum at the orthogonal polarization. For WDMsiganls, these eigen state wavelengths are the ITU values and the twosets of the eigen states interleaver each other. The crystals used inthe filter can be designed with different lengths and with differentmaterials. These crystals can be placed in series to achieve variouswavelength interleaving spectrum, such as flat top, and also tocompensate temperature as well as dispersion effects.

[0029] The present invention will be further described in terms ofseveral optical wavelength switch embodiments having specific componentsand having specific configurations.

[0030] Two Stage Wavelength Selective Switch

[0031]FIG. 2 schematically depicts an embodiment of a 3 ports two-stageinventive non-mechanical optical wavelength switch. The inventionrelates to an optical switch comprising several optical components whichare optically coupled along the longitudinal axis: a pair of beamdisplacer/combiner 12 and 13 that displaces at least one optical beaminto two polarized component beams and combines at least two polarizedcomponent beams to form an optical beam; a pair of two-aperture halfwaveplate 14 and 15, for rotating the polarization of the beams such thatboth beams have the same polarization state or rotating two parallelpolarization beams into orthogonal polarizations; a pair of electricallycontrollable rotator 16 and 17 for rotating the polarization orientationof the polarized component beams upon an electrical signal to directbeams between two paths; a pair of birefringent filters 18 and 21 thatselectively rotate the polarization of wavelengths to produce filteredsignals; a polarization walk-off crystal 20 which shifts one set of thepolarization beam laterally to form a second path, and a beam angledeflector 19 that deflects all beams with a correction angle such thatboth optical paths are coupled into the dual collimators that have anangular between the two beam propagations. The switch has two stagecascaded configuration.

[0032] To more particularly illustrate the method and system inaccordance with the present invention, refer now to FIGS. 3 and 4depicting one embodiment of a three ports two-stage (1×2) opticalwavelength switch. FIG. 3A depicts a top cross-section view of opticalswitch and FIG. 3B depicts a side cross-section view of the opticalswitch. FIG. 4 further depicts the propagating beams' polarizationstates as they exit each component. A first optical fiber 1 is insertedinto a first collimator 10. Opposite first fiber 1, a second opticalfiber 2 is inserted into a second collimator 11 and a third opticalfiber 3 is inserted into the same collimator 11 adjacent to fiber 2, sothat fiber 2 and fiber 3 are parallel. Beam propagations from fiber 2and fiber 3 has an angle with respect to the y-axis caused by the focuslens inside the collimator.

[0033] As shown in FIG. 4, beam 30 that contains the full spectrum ofdata passes through a first birefringent block 12 and is thereby dividedinto two beams having orthogonal polarizations, specifically beams 30Aand 30B. The length of birefringent block 12 is adjusted to obtain aspatial separation between beams 30A and 30B, which permits to pass themthrough independent optical elements, such as two-aperture waveplate 14.Beam 30A then enter a first halfwave plate 14 which rotates its plane ofpolarization by 45° clockwise. Beam 30B enter another part of the firsthalfwave plate 14 which rotates the plane of polarization by 45°counterclockwise. Therefore, halfwave plate 14 renders the polarizationsof beam 30A and 30B parallel to each other.

[0034] Considering a first switching state in which light path of thespectral band that contains λ₁ is from port 1 to port 2 and thecomplementary spectral band that contains λ₂ is guided out through port3, as indicated in FIG. 4A. In this light path sate, both beams enterthe first electrically controllable polarization rotator 16 whichrotates the plane of polarization by 45° clockwise with a correspondingelectrical control current. The beams then pass through a birefringentfilter 18 which rotates the polarization of λ₂ spectrum band by 90° butpasses the spectrum band containing λ₁ unaltered. Beam 30 is now furtherdecomposed into two sets of orthogonally polarized beams: beams 31A and31B for the λ₁ spectrum band and beams 32A and 32B for the λ₂ spectrumband, as shown in FIG. 4A. The two spectrum bands are subsequentlyspatially separated by a birefringent walk-off element 20 which changesthe propagation of 32A and 32B of λ₂ spectrum band with a spatialdisplacement in x-axis.

[0035] All the beams than pass through the second stage birefringentfilter 21 which rotates the polarization of beams 32A and 32B by 90° butpasses beams 31A and 31B unaltered.

[0036] At this point both beams propagate parallel to the longitudinaly-axis which need to be bent at an angle θ with respect to the y-axis inorder to be efficiently coupled into the dual fiber collimator 11. Apolarization-independent light-bending device 19 corrects this angle ofpropagation.

[0037] All beams than pass through the second electrically controllablepolarization rotator 17, which rotates their polarization by 45°counterclockwise by applying an associated electrical current flow. Allfour beams further enter a halfwave plate 15, which selectively rotatesthe polarization of 32B and 31B by 45° counterclockwise and rotates 32Aand 32A by 45° clockwise. Block 13 subsequently combines orthogonallypolarized beams 31A and 31B to form a single beam 31 that is alsofocused on optical fiber 3. Similarly, block 13 combines beams 32A and32B to form a single beam 32 that is focused on optical fiber 2.Therefore an optical path from fiber port 1 to fiber 2 for the λ₁wavelength band and another optical path from fiber port 1 to fiber 2for the λ₂ wavelength band are established, when an appropriate controlsignal is applied to both electrically controllable Faraday rotators 16and 17.

[0038] Next, considering a second wavelength switching state in whichlight path for λ₁ spectral band is from port 1 to port 3 and for thecomplementary λ₂ spectral band is from port 1 to port 2, as indicated inFIG. 4B. In this light path sate, both beams 30A and 30B enter the firstcontrollable Faraday rotator 16 which rotates the plane of polarizationby 45° counterclockwise with a corresponding current, rendering them inthe horizontal direction, as seen in FIG. 4B. Birefringent filter 18rotates the polarization of λ₂ spectrum band by 90° but does not changeλ₁ spectrum band. The two spectrum bands are subsequently spatiallyseparated by a birefringent walk-off element 20 which alters thepropagation of λ₁ spectrum band with a spatial displacement. Beam 30 isthereby further divided into four beams: 31A and 31B for the λ₁ spectrumband and 32A and 32B for the λ₁ spectrum band.

[0039] All four beams than pass through the second stage birefringentfilter 21 which rotates the polarization of beams 31A and 31B by 90° butpasses beams 32A and 32B unaltered. A polarization-independentlight-guiding device 19 further bends the beams an angle θ with respectto the y-axis to facilitate coupling into the dual fiber collimator 11.

[0040] All beams than pass through the second electrically controllablepolarization rotator 17, which rotates their polarization by 45°clockwise by applying an associated electrical current flow. All fourbeams further enter a halfwave plate 15, which selectively rotates thepolarization of 32B and 31B by 45° counterclockwise and rotates 32A and32A by 45° clockwise. Block 13 subsequently combines orthogonallypolarized beams 31A and 31B to form a single beam 31 that is alsofocused on optical fiber 3. Similarly, block 13 combines beams 32A and32B to form a single beam 32 that is focused on optical fiber 2.Therefore an optical path from fiber port 1 to fiber 2 for the λ₂wavelength band and another optical path from fiber port 1 to fiber 3for the λ₁ wavelength band are established, when a control signal thatis opposite to that of the first switching state is applied to bothFaraday rotators 16 and 17.

[0041] The above embodiment is a nonreciprocal device using electricallycontrollable polarization rotators 16 and 17 of 45° magneto-opticFaraday rotators. Another preferable embodiment of FIG. 2 is areciprocal wavelength switch. The reciprocal embodiment requires straitforward modifying the halfwave plate 14 and 15 and using controllablepolarization rotators 16 and 17 of 90° rotation in the abovenonreciprocal embodiment. Both magneto-optic Faraday rotators andelectro-optic retarders can be used to construct the 90° rotator 16 and17 in the reciprocal wavelength switch embodiment. As described in ourpending U.S. patent application, an inventive reciprocal Faraday rotatorthat comprises a switchable first 45° garnet and a second permanent 45°polarization rotation garnet is applicable to be used as electricallycontrollable polarization rotators 16 and 17 in a bi-directionalwavelength switch embodiment. The combined Faraday rotator rotates lightpolarization between 0° when the two garnet rotations cancel each otherand 90° when the two garnet rotations are in the same direction. Anelectro-optic rotator configuration with side electrodes described inour pending patent is also applicable here to be used as electricallycontrollable polarization rotators 16 and 17 in the reciprocalwavelength switch embodiment.

[0042] In one embodiment, the Faraday polarization rotator comprisesyttrium-iron-garnet (YIG), or Bi-added thick film crystals with a lowfield of saturation, such as less than 200(Oe) to reduce powerconsumption. One example of such materials is bismuth-substituted rareearth iron garnet single crystal system represented by a chemicalformula (GdRBi)₃(FeGaAl)₅O₁₂, where R denotes at least one elementselected from the group consisting of yttrium (Y), ytterbium (Yb) andlutetium (Lu). The electromagnet coupled to Faraday rotator comprisesprimarily cupper coils. Ion alloys are often incorporated into theelectromagnet to improve electrically induced magnetic field strength.Semi-hard magnetic metallic alloys can be used to achieve latchingperformance, although this is not essential for self-latching typegarnets. Therefore, the inventive switch requires only current pulse toswitch optical path from one to another by reversing the polarity andlatches to the previous switching state even when the current isremoved.

[0043] The general requirement for the electro-optic phase retarder usedin the inventive switches is that, when a voltage is applied, apolarization rotation of 90° or ±45° is produced. Preferably, thematerial has a high electro-optic coefficient to reduce operatingvoltages to less than 500 volts, good thermal stability, and goodtransparency at the wavelength of interest, e.g., between 1200 nm and1600 nm. These requirements are satisfied by a class of ferroelectriccomplex oxides which have a Curie temperature less than about 600° C.,so that electro-optic coefficients are high in the operation temperaturerange. Example material systems are: a solid solution of lead manganeseniobate and lead tantalate (PMN—PT) and a solid solution of lead niobatezirconate and lead tantalate (PNZ—PT), lead manganese niobate (PMN),lanthanum modified PZT (PLZT), and More members of this class may bediscovered in the future. It is particularly preferable to usesingle-crystal of the said class of ferroelectric materials, providinggood repeatability and temperature independent operation. Another familyof electro-optic materials applicable to the inventive switches is newsolid organic materials, such as polymers and organic crystals withlarge electro-optic effect. Solid organic electro-optic materials havean advantage of higher switching speed due to their relatively smallerdielectric constant.

[0044] There are many methods to make light-bending device 19. Oneembodiment of device 19 consists of a tapered glass prism, whose angleis adjusted so that beams enter from fiber port 2 and 3 are renderedparallel to the y-axis as the beams exit device 19. Other shapes andconstructions of prisms can also perform the same function. In anotherembodiment, the light guiding device 19 can be constructed using twotapered birefringent plates usually from the same birefringent materialto change angle of propagation. Two such examples are Wollaston type andRochon type prisms.

[0045] The above device is a specific embodiment. However, one ofordinary skill in the art will readily recognize that this method andsystem will operate effectively for other components having similarproperties, other configurations, and other relationships betweencomponents.

[0046] Reflection Mode Wavelength Selective Switch

[0047] An alternative embodiment of the present invention is a foldedthree ports optical wavelength selective switch configuration, whichuses fewer and shorter components than the strait embodiment. FIG. 5depicts a specific nonreciprocal dual stage reflection mode (1×2)wavelength selective switch configuration. By use of a right angle prism22, this reflection mode switch essentially folds the straight switch inFIG. 2 from the center. Therefore, the reflection configurationadvantageously eliminates the need for elements 21, 17, 15, and 13 aswell as shortens the lengths of birefringent elements 18 and 20 by halfdue to the double passes. A dove type prism type position displacer 23is incorporated here to provide larger separating between collimator 10and 11 for ease of manufacturing. A plate 24 is also added to compensatethe traveling distance difference between the two polarizationcomponents caused by birefringent crystal 12. In this embodiment theswitchable polarization rotator 16 is a 45° Faraday garnet rotator. Theoperation principle can be easily understood in the same way as theabove embodiments by following the ray traces illustrated in FIG. 5.

[0048]FIG. 6 depicts an example of bi-directional single-stagereflection-mode wavelength switch. In this embodiment the switchablepolarization rotator 16 is a 90° rotator of Faraday garnets or anelectro-optic crystal, similar to the strait version discussed above. Inthis configuration, 14 comprises a halfwave 90° rotator bottom apertureand a polarization mode-dispersion compensation plate top aperture. Thisinventive configuration uses less components than the above embodiments.

[0049] Reflection Mode Wavelength Interleaver

[0050] The inventive device can also be configured as a passive opticalwavelength interleaver. FIG. 7 depicts a passive reflection interleaverembodiment. This inventive device uses fewer components and has increasealignment tolerance than prior arts. Therefore, it is easier to beproduced and its cost is lower. The operation principle can be easilyunderstood by following the ray traces illustrated in FIG. 7, the sameway as described in the above sections.

[0051] Reflection Mode Wavelength Independent Switch

[0052] The inventive device can be further configured to function aswavelength-independent optical light path switches by simply removingwavelength filter 18. FIG. 8 depicts a bi-directional 1×2 optical switchembodiment. A light beam 1 is launched through first collimator 10,displaced spatially by a prism 23, so that alignments of collimator 10and 11 are made easier. The input beam is then decomposed into twoorthogonally polarized components and spatially separated by walk-offcrystal 12. Their polarizations are consequently rotated by halfwaveplate 14 rendering them parallel in the z direction. Considering a firstswitching state in which light path is from 1 to 2, as indicated bysolid beam propagation line in FIG. 8. In this light path sate,electrically controllable polarization rotator 16 rotates the plane ofpolarization by 0°. The two beams then pass a birefringent walk-offelement 20 unaltered. Right-angle prism 22 polarization-independentlyreflects back the beam with a displacement in x direction. The reflectedbeams pass 20 without change but are bended by 19 at an angle thatmatches the coupling angle of second collimator 11. Again, the reflectedbeams pass 16 without rotation. Halfwave plate 14 renders the parallelpolarized reflected beams orthogonal and block 12 combines the two beamsto form a single beam that is focused to optical fiber 2 mounted incollimator 11. Therefore an optical path from fiber port 1 to fiber 2 isestablished, when no rotation is applied to rotator 16.

[0053] Next, considering a second switching state in which light path isfrom port 1 to port 3, as shown in FIG. 8 by the dotted beam propagationline. Similarly, fiber 1 emits a light beam that becomes two verticallypolarized beams after 14. In this light path sate, electricallycontrollable polarization rotator 16 rotates the plane of polarizationby 90°. The two horizontally polarized beams are then displaced at adistance in x direction by passing birefringent walk-off element 20.Right-angle prism 22 reflects back the beam with another displacement inx direction. The reflected beams pass 20 with another furtherdisplacement in x direction and are bended by 19 at an angle. Again, thereflected beams pass 16 with a second stage 90° rotation. Halfwave plate14 renders the parallel polarized beams orthogonal and block 12 combinesthe two reflected beams to form a single beam that is focused to opticalfiber 3. Therefore an optical path from fiber port 1 to fiber 3 isestablished, when a 90° rotation is applied to rotator 16.

[0054] The above descriptions of the 1×2 embodiments are very specificexamples. It will be apparent to a person of average skill in the artthat many variations of the switch are possible within the scope of theinvention. Accordingly, the scope of the invention should be determinedby the following claims and their legal equivalents.

What is claimed is:
 1. An optical switch for selectively directing lightwith a certain set of spectrum from a first fiber to a second fiber orto a third fiber and directing light with another set of spectrum fromsaid first fiber to said third fiber or to said second fiber, saidsecond fiber and said third fiber being located adjacent to each otheralong a longitudinal axis, said optical switch comprising along saidlongitudinal axis in sequence from said first fiber to said second andthird fibers: a) a first lens for guiding light from said first fiberand to said second or third fibers; b) a first block of birefringentmaterial for separating and combining mutually orthogonal polarizations;c) a first compound half-wave plate for rendering mutually parallelpolarizations orthogonal and mutually orthogonal polarizations parallel;d) a first compound polarization rotator whose polarizations rotationcan be electrically controlled; e) a fist wavelength filter whosepolarization rotation is wavelength dependent; f) apolarization-dependent beam path deflector; g) a second wavelengthfilter whose polarization rotation is wavelength dependent; h) apolarization-independent beam angle corrector; i) a second compoundpolarization rotator whose polarizations rotation can be electricallycontrolled; j) a second compound half-wave plate for rendering mutuallyparallel polarizations orthogonal and mutually orthogonal polarizationsparallel; k) a second block of birefringent material for separating andcombining mutually orthogonal polarizations; and l) a second lens forguiding light to said second or third fiber from said first fiber,wherein said second fiber and third fiber are placed adjacent to eachother to form a dual collimator and exit said second lens at an angle θwith respect to said longitudinal axis.
 2. The optical switch of claim 1wherein said beam corrector is a glass prism that provides a beam areceiving angle for fiber in dual fiber collimator.
 3. The opticalswitch of claim 1 wherein said polarization-dependent beam pathdeflector comprises two tapered birefringent plates.
 4. The opticalswitch of claim 1 wherein said first and second compound polarizationrotators comprise a 45° Faraday rotator, and said Faraday rotator iscoupled to electromagnets.
 5. The optical switch of claim 1 wherein saidfirst and second compound polarization rotators comprise a 90° Faradayrotator.
 6. A compound polarization rotator assembly according to claim5 comprises a first switchable 45° Faraday rotator that is coupled to anelectromagnet and a second permanent 45° Faraday rotator, the saidsecond Faraday rotator comprises either a latching garnet plate or agarnet plate saturated by a permanent magnet.
 7. A Faraday rotatorassembly according to claims 4 and 6, wherein said magnetic fieldapplying means is formed by a coil and an electromagnet formed ofsemi-hard magnetic material.
 8. The optical switch of claim 1 whereinsaid first and second compound polarization rotators are selected from aclass of garnet materials characterized by having a saturation field ofless than 500 Oe.
 9. The optical switch of claim 1 wherein said firstand second compound polarization rotators comprise an electro-opticretarder.
 10. The optical switch of claim 1 wherein said first andsecond compound polarization rotators are selected from a class offerroelectric materials characterized by having a Curie temperature ofless than about 600° C. and having a Vπ of less than about 600V.
 11. Theoptical switch of claim 1 wherein said first and second compoundpolarization rotators are selected from a class of solid organicmaterials characterized by having a Vπ of less than about 600V.
 12. Theoptical switch of claim 1 wherein said first and second blocks ofbirefringent material, said beam deflector, said tapered plates, andsaid wavelength filters comprise a material selected from the groupconsisting of rutile, calcite, and yttrium orthovanadate.
 13. Areflection mode optical wavelength switch for selectively directinglight with a certain set of spectrum from a first fiber to a secondfiber or to a third fiber and directing light with another set ofspectrum from said first fiber to said third fiber or to said secondfiber, on the same side of said first fiber said second fiber and saidthird fiber being located adjacent to each other along a longitudinalaxis, said optical switch comprising along said longitudinal axis insequence: a) a first lens for guiding light from said first fiber and tosaid second or third fibers; b) a second lens for guiding light to saidsecond or third fiber from said first fiber, wherein said second fiberand third fiber are placed adjacent to each other to form a dualcollimator and exit said second lens at an angle with respect to saidlongitudinal axis; c) a block of birefringent material for separatingand combining mutually orthogonal polarizations; d) a compound half-waveplate for rendering mutually parallel polarizations orthogonal andmutually orthogonal polarizations parallel; e) a compound polarizationrotator whose polarizations rotation can be electrically controlled; f)a polarization-independent beam angle corrector; g) a wavelength filter;h) a polarization-dependent beam path deflector; i) a prism reflector.14. The said prism deflector of claim 13 is a total reflection rightangle prim.
 15. The reflection optical switch of claim 13 furthercomprises a beam displacement prism located in front of said first lensfor increasing the separation between said first lens and said secondlens.
 16. The reflection optical switch of claim 13 wherein the saidcompound polarization rotator is a 45° rotator.
 17. The reflectionoptical switch of claim 13 wherein the said compound polarizationrotator is a 90° rotator.
 18. The reflection optical switch of claim 13wherein the said compound halfwave plate further comprises acompensation plate that is configured to compensate for optical pathlength difference between an ordinary ray and an extraordinary ray inthe said block of birefringent material.
 19. A reflection mode opticalwavelength interleaver for directing light with a certain set ofspectrum from a first fiber to a second fiber or to a third fiber anddirecting light with another set of spectrum from said first fiber tosaid third fiber or to said second fiber, on the same side of said firstfiber said second fiber and said third fiber being located adjacent toeach other along a longitudinal axis, said optical switch comprisingalong said longitudinal axis in sequence: a) a first lens for guidinglight from said first fiber and to said second or third fibers; b) asecond lens for guiding light to said second or third fiber from saidfirst fiber, wherein said second fiber and third fiber are placedadjacent to each other to form a dual collimator and exit said secondlens at an angle with respect to said longitudinal axis; c) a block ofbirefringent material for separating and combining mutually orthogonalpolarizations; d) a compound half-wave plate for rendering mutuallyparallel polarizations orthogonal and mutually orthogonal polarizationsparallel; e) a polarization-independent beam angle corrector; f) awavelength filter; g) a polarization-dependent beam path deflector; h) aprism reflector.
 20. A reflection optical switch for directing lightfrom a first fiber to a second fiber or to a third fiber, on the sameside of said first fiber said second fiber and said third fiber beinglocated adjacent to each other along a longitudinal axis, said opticalswitch comprising along said longitudinal axis in sequence from saidfirst fiber to said second and third fibers: a) a first lens for guidinglight from said first fiber and to said second or third fibers; b) ablock of birefringent material for separating and combining mutuallyorthogonal polarizations; c) a compound half-wave plate for renderingmutually parallel polarizations orthogonal and mutually orthogonalpolarizations parallel; d) a compound polarization rotator whosepolarizations rotation can be electrically controlled; e) apolarization-independent beam angle corrector; f) apolarization-dependent beam path deflector; g) a prism reflector. 21.The reflection optical switch of claim 20 wherein the said compoundpolarization rotator is a 45° rotator.
 22. The reflection optical switchof claim 20 wherein the said compound polarization rotator is a 90°rotator.